I was looking through the Eberron Campaign Setting and I noticed that the book specifically said 'Although living constructs do not need to sleep, a warforged wizard must rest for 8 hours before preparing spells.' This is the only reference I could find to warforged being forced to rest. The line does not say 'Warforged prepared casters must rest for 8 hours before preparing spells', or even 'Warforged magic-users must rest for eight hours to regain their spells.'
So, by RAW, Warforged don't necessarily ever need to rest unless they are Wizards. Does this mean that a Warforged Sorcerer only has to spend the 15 minutes dictated in the PHB to regain his spells? Would a Warforged Druid only need to spend an hour to prepare new spells? If so, what about Wild Shape uses? Are those effectively infinite?
While I'm at it, what exactly constitutes 'rest' as the book specifies? Must the warforge lay down and become inert? Is just standing in place sufficient? What about walking? For a creature that is effectively an automaton and one that never tires at that, I wouldn't think that walking would keep it from resting.
My guess is that the writer wanted a warforged caster to sit immobile for eight hours as any other race would, but I don't see any text really supporting that.